Conventionally, varactor diodes may be used for tuning oscillator circuits. Varactor diodes are diodes that have a particularly pronounced dependence of their blocking capacitance on the electrical voltage that is applied. By applying an electrical direct voltage to the varactor diode, this effect may be used to set its capacitance and thereby the resonant frequency or other parameters of an oscillator circuit.
In order to be able to cover a large frequency range, using these oscillator circuits or filters, it may be advantageous if the varactor diodes used have a large capacitance ratio or a large capacitance swing, that is, the relative difference between the smallest and the largest capacitance value that may be set is particularly great. In particular, the entire available capacitance swing of the varactor diodes should be achievable even when having low control voltages applied, so that the entire range may be utilized in the case of battery-operated units. The same also applies in connection with integrated circuits, which often have to be supplied, in the course of smaller semiconductor structures, with ever lower operating voltages, so that a subsequent voltage increase to achieve higher control voltages for the varactor diode would usually be too costly.
Since the capacitance of conventional varactor diodes changes very rapidly with the voltage controlling them, varactor diodes not only react to the direct voltage applied, but also to the signal voltage applied to the oscillator circuit. In the case of large signal voltage levels, the capacitance of the varactor diode accordingly fluctuates in time with the useful signal, that is to be processed in the oscillator circuit, so strongly that distortions in the signal come about, especially intermodulation. In this connection, interference signal components may be created that impair the useful signal, and cannot be subsequently filtered out. This effect occurs particularly strongly in diodes having a large capacitance ratio, since, in this instance, even smaller signal voltages are sufficient in order to generate a no longer acceptable interference signal strength.
Conventionally, to correct this two varactor diodes having opposite polarity are connected in series (FIG. 5). In this connection, at both diodes respectively the direct voltage becomes effective for the capacitance change, but only half the alternating voltage, which leads to correspondingly smaller interference products. In addition, the opposite connection has the effect that each of the half waves acts in a capacitance-increasing manner on one of the diodes, and in a capacitance-decreasing manner on the respective other diode, which also acts counter to the creation of interference products.